This invention relates generally to fasteners, and more particularly concerns blind fastener connection to work, in such manner as to provide excellent corrosion resistance and high shear strength of work panels.
Blind fasteners are commonly characterized as employing a tubular shank and a tapered annular head, as well as a stem which is retractable at the exposed or front side of the work in order to expand locking structure at the "blind" or unexposed side of the work. Such expansion serves to lock the fastener to the work. Any excess exposed extent of the stem may then be severed. Use of such has not always been advisable, because of the larger holes required in the work due to increased diameter of the head.
Also, in most instances excessive recessing of the fastener heads was considered necessary for engagement by tooling holding the head and shank against rotation upon rotary retraction of the threaded stem. In addition, prior blind fasteners tended to loosen and fall-out of position in response to structural vibration, resulting in decreased fatigue life and destruction of sealing engagement of the head and countersink bore, so that corrosive fluid can then gain access to the interface, corroding the connection. Also, prior blind fasteners are in general not applicable to original design installation in shear load applications where thin panels are used, because required head heights result in countersink recesses that are near the knife edge condition, reducing shear bearing area which results in lower fatigue strength and loosening of the fastener.